1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus and method for analyzing an intravascular state, and more particularly to an ultrasonic diagnostic apparatus and method for diagnosing the property and state of an intravascular sclerotic nidus called “plaque” according to an intravascular echoing process.
Signs and symptoms that result in an observation of acute ischemia, such as unstable angina and acute myocardial infarction, are called acute coronary syndrome. Heretofore, it has been considered that the acute coronary syndrome is caused by a constriction or obstruction in a blood vessel that is produced by a gradual accumulation of plaque (sclerotic nidus) within the blood vessel over many years.
In recent years, however, a certain type of plaque that is formed in a blood vessel is thought to be responsible for the development of acute coronary syndrome. Generally, a plaque comprises a soft atheromatous lipid component and a relatively hard fibrous capsule (fibrous component). A plaque having a higher lipid content is a vulnerable plaque that can easily be broken by even a small stimulus. A plaque having a lower lipid content is a plaque that is less liable to be broken. The vulnerable plaque with the higher lipid content is softer and more deformable than the plaque with the lower lipid content. The former plaque is called a soft plaque, whereas the latter plaque is called a hard plaque. When the vulnerable soft plaque is broken, it produces a thrombus, causing a constriction or obstruction in the blood vessel which tends to bring about the acute coronary syndrome.
For preventing and appropriately treating the acute coronary syndrome, it is necessary to establish a technique to evaluate the property and state of plaques. Particularly, there is a need for adequately determining whether a plaque formed in a blood vessel is a vulnerable soft plaque or a hard plaque.
For determining the property and state of plaques, it is better to use an intravascular echoing process than to use an X-ray angiographic process. According to the intravascular echoing process, a catheter having an ultrasonic probe on its end is inserted into a blood vessel. The ultrasonic probe transmits an ultrasonic wave while circumferentially scanning a location to be evaluated (radial scanning), and receives a reflected wave to produce a received signal. The amplitude of the received signal is modulated in luminance to produce a tomographic image of the blood vessel at the evaluated location.
In actual clinic sites, the property and state of a plaque in the blood vessel are analyzed based on the produced tomographic image to regard a region of high luminance as a fibrous component of the plaque and also to regard a region of low luminance as a lipid component of the plaque. For example, a plaque containing 80% or more of a region of high luminance is regarded as a hard plaque, and a plaque containing 80% or more of a region of low luminance is regarded as a soft plaque. In this manner, a pseudo evaluation of the property and state of plaques is made.
However, the direct relationship between luminance and plaque state is not strong enough. It is difficult to analyze the property and state of plaques accurately in detail according to the analyzing process based solely on luminance. There has been a demand for a technique to directly analyze the property and state of plaques by determining dynamic characteristics of plaques. To meet such a demand, there have been proposed various techniques to determine dynamic characteristics of plaques, such as a distortion value and a modulus of elasticity, using ultrasonic energy
Japanese Patent Laid-open No. 2000-229078 discloses a technique for tracking the position of a location to be evaluated in a blood vessel, calculating the modulus of elasticity of the wall of the blood vessel, and evaluating the property and state of a plaque in the blood vessel.
Japanese Patent Laid-open No. Hei 8-10260 discloses a technique for allocating partial regions of two images obtained at respective two times, determining a complex conjugate product, determining a displacement from the gradient of the phase of the complex conjugate product, and evaluating the rigidity of a tissue in an examined sample.
Japanese Patent Laid-open No. Hei 5-317313 reveals a technique for calculating the absolute modulus of elasticity at a location to be evaluated in a blood vessel based on a change in the distance between two particular points and the blood pressure in the blood vessel. This document also discloses a technique for displaying a tomographic image of a blood vessel with a different hue added depending on the difference between the moduli of elasticity that are determined at respective locations to be evaluated in the blood vessel.
Japanese Patent No. 3182479 reveals a technique for determining a displaying a ratio of moduli of elasticity (indirect modulus of elasticity) which represent respective elastic levels at an observed point and a reference point, rather than calculating an absolute modulus of elasticity.
The above conventional techniques calculate a distortion value and/or a modulus of elasticity to evaluate the hardness of a location to be evaluated in a blood vessel. Actually, the distortion values of a blood vessel wall and a plaque change periodically because the cross-sectional area of the lumen of the blood vessel changes periodically due to blood pressure changes caused by cardiac beats. Specifically, when the blood vessel is contracted and expanded, the distortion values change greatly, and when the blood vessel switches from a contracted state to an expanded state and also from an expanded state to a contracted state, the distortion values do not change significantly.
The blood vessel wall and the plaque are essentially elastic bodies that are dynamically nonlinear. Stresses in the blood vessel wall and the distortion values are nonlinearly related to each other. Because of the nonlinearity, the moduli of elasticity of the blood vessel wall and the plaque change with time depending on the contracted and expanded stages of the blood vessel. In addition, since the blood vessel wall and the plaque are viscous, they need to be treated as viscoelastic bodies.
The conventional techniques referred to above do not analyze the state of a location to be evaluated based on a time-dependent change of a chronological sequence of distortion values and moduli of elasticity. Therefore, it has been impossible with the conventional techniques to analyze the state of a location to be evaluated, i.e., the property and state of a plaque, in view of the dynamic nonlinearity and viscoelasticity of the blood vessel wall and the plaque.